Búsqueda de nueva física a través de la producción de resonancias escalares de baja masa en colisiones $pp$ del LHC con el detector ATLAS

Under the validity of certain BSM theories, like models involving Axion-Like particles, during a proton-proton collision at the LHC, a low-mass pseudoscalar resonance X can be produced, coupling mainly with third-generation SM particles. These resonances could be detected by ATLAS, produced in association with a pair of top and anti-top quarks, and decaying to a pair of tau leptons, considered to decay hadronically ($t\bar{t}(X \to \tau_{\text{had}}\tau_{\text{had}})$). The $X \to \tau\tau$ decay channel presents technical difficulties in the range of masses between 20 and 60 GeV since the individual tau leptons can’t be resolved in the detector due to a low angular separation. The main challenges in this experimental search are the reconstruction and characterization of the X particle’s boosted decays for their proper identification, and the estimation of the SM background events that could contaminate the New Physics signal. With these objectives, in this thesis, we will define control and validation regions for the dominant $t\bar{t}$ background, estimating the normalization factors by comparing the MC simulations and 2017 data recorded by ATLAS.